But there is effective geographical isolation.
If you are from Europe (for example) how many Mauries (SP?)
do you know ... or Bantu, or Inuit, or ....
We have geographic isolation and we also have cultural isolation
in many cases. Some individuals refuse out-right to take partners
outside of their own race, religion, or what-have-you.
Sexual selection is also a part of Darwin's original concept.
While we are able to travel, few do in order to meet diverse
mates. The 'slow-down'/stop to evolution that we seem to have
may be more to do with the apparent genetic bottleneck that
humans have encountered sometime in recent (geological)
time.